Activation Energy For Boundary Migration Research Articles

Grain boundary kinetics in magnesium alloys from first principles

Grain boundary migration in magnesium alloys has been studied using quantum mechanical calculations implementing the nudged elastic band method. Four crystallographically different boundaries were examined: two twin boundaries and two general grain boundaries that showed no crystallographic symmetry across the boundary plane. The activation energies for boundary migration were determined from the minimum energy pathways, and these energies were consistent with experimental values. It was found that the activation energy is linearly related to the coordination number of the boundaries. This indicates that boundaries with lower coordination numbers showed smaller activation energies and thus higher mobilities than the more orderly boundaries with larger coordination number and larger activation energies. The effect of solutes at the boundary was also studied, and it was found that most solutes with low co-ordination number decreased the activation energy for boundary migration, but the effect of solutes on boundaries with high coordination number was strongly dependent on the solute chemistry.

The microstructural evolution and grain growth kinetics of TZ20 alloy during isothermal annealing

The microstructural evolution and grain growth kinetics of the hot rolled Ti–20Zr–6.5Al–4V (wt.%, TZ20) alloy during subsequent isothermal annealing were investigated. The XRD results show that the examined alloy is composed of α and β phases after annealing at temperatures from 750 °C to 850°C. The SEM results show that the microstructure of all annealed specimens is typical basketweave structure with interlaced α plates embedded in β matrix. Thickness growth kinetics of α plate in TZ20 alloy was investigated by designing various heat treatments. The grain growth exponent n, pre-exponential factor K0 and activation energy of boundary migration E of TZ20 alloy during isothermal annealing were determined. Finally, the relationship between thickness increment Δh, annealing temperature T and holding time t of isothermal annealed TZ20 alloy has been deduced. Mechanical properties of the examined alloy with various annealing treatments were also investigated. Tensile results show that the strength changes in the range of 1120MPa to 1246MPa, and the ductility in term of elongation varies from 9.1% to 14.3%. The relation between mechanical properties and microstructure was also discussed. This work is of great significance to guide the manufacture and to promote practical applications of TiZr-based alloys.

Recrystallisation in a cold drawn low cost beta titanium alloy during rapid resistance heating

Abstract The microstructure and micro-texture evolution in an 80% and 90% cold drawn LCB-Ti alloy subjected to rapid annealing was analysed via electron back-scattering diffraction. The partially recrystallised microstructures were deconstructed into four fractions comprising deformed, recovered, newly nucleated and growing grains. The specific interfacial areas of the various recrystallisation fronts were used to estimate the activation energy for boundary migration. It is inferred that diffusion controlled boundary migration is operational during rapid heating as the apparent activation energies for the deformed–recrystallised interfaces are close to the activation energy for the self-diffusion of β-Ti. Compared to the newly nucleated grains, the increase in the apparent activation energy of the growing grains suggests solute drag effects during the growth stage. The recrystallisation micro-textures of both cold drawn reductions exhibit varying extents of high and low energy discontinuous recrystallisation and continuous recrystallisation.

The effect of solutes on grain boundary mobility during recrystallization and grain growth in some single-phase aluminium alloys

The effect of solutes (Si, Mn, Mg) in quantities typical of commercial aluminium alloys, on grain boundary mobility in aluminium, has been investigated with in situ annealing and electron backscattered diffraction in the SEM, and grain growth experiments. The in situ experiments provided information on the migration of the high mobility tilt boundaries of misorientations close to 40°〈1 1 1〉. Grain growth experiments were used to investigate boundary migration in alloys of high solute content (1–5wt%Mg), and a comparison between the in situ and bulk experiments is made. The relationship between boundary velocity and driving pressure was found to be linear in all cases, and the activation energies for boundary migration were higher than those controlled by lattice diffusion of the solutes at higher solute concentrations.

Grain boundary mobilities during recrystallization of Al–Mn alloys as measured by in situ annealing experiments

The influence of Mn on the mobilities of grain boundaries during recrystallization of Al–0.1 and –0.3 wt.% Mn alloys has been characterized by in situ SEM annealing experiments. Polycrystals of high purity, single-phase Al–Mn alloys were deformed in channel-die plane strain compression at room temperature to strains of 1.3. The specimens were in situ annealed in an SEM/EBSD in order to measure grain boundary mobilities at temperatures between 200 and 450 °C. Stable “loaded” boundary migration was observed in the 0.1 and 0.3% Mn alloys. However, unstable, partially “free”, boundary migration could also be found in the 0.1% alloy. The mobilities, deduced from the migration rates and the stored energies, were consistent with the solute drag theories of Cahn, Lücke and Stüwe. The diffusion rates controlling the solute drag were of the same order for both theories and the activation energy for boundary migration was found to be intermediate between that of solute diffusion in the lattice and along the grain boundaries.

High Angle Grain Boundary Mobilities in a High Purity Single-Phase Al-0.1 wt% Mn Alloy

The migration rates (V) of “random” high angle grain boundaries (HAGB) during annealing of a cold deformed (e=1.3) high purity Al-0.1wt% Mn alloy were determined using a combination of in-situ annealing and EBSD in the SEM at temperatures between 200 and 330°C. The SEM heating stage used for these experiments is described and results on the local recrystallization kinetics of the Al-Mn alloy are presented. For this, the local stored energies (P) were determined by subgrain size and misorientation analyses to give the boundary mobilities (M) through the standard V = M.P equation. The solute drag “force” was analysed with the atomistic model of Lücke and Stüwe (1971) for a “loaded” boundary and used to estimate the diffusion rates in the above temperature range. The activation energies for boundary migration were found to be consistent with those of solute atoms moving behind the grain boundary, i.e. intermediate between the values for bulk and boundary diffusion of Mn.

Kink model for extended defect migration in the presence of diffusing impurities: theory and simulation

The mobility of extended defects in solids (e.g., grain boundaries, anti-phase boundaries, dislocations, ferroelectric and magnetic domain walls) is often controlled by their interactions with impurities that can move diffusively. In this paper, we develop a theoretical model for extended defect migration in the presence of diffusing impurities which is valid in cases where impurity drag is significant. Model predictions of boundary velocity versus driving force, bulk impurity concentration, impurity diffusivity and temperature were shown to be in good agreement with kinetic Monte Carlo simulations based on an Ising model. At low temperatures and/or sufficiently large bulk concentrations, the kink model predicts that the boundary mobility is independent of the bulk impurity concentration. The activation energy for boundary migration is shown to depend on the formation energy of kinks on the boundary, the heat of segregation and the activation energy for bulk diffusion. The dependence on the kink formation energy remains even in the strong impurity drag limit. The present model is compared with earlier continuum models.

Misorientation dependence of intrinsic grain boundary mobility: simulation and experiment

Both experimental and atomistic simulation measurements of grain boundary mobility were made as a function of temperature and boundary misorientation using the same geometry that ensures steady-state, curvature-driven boundary migration. Molecular dynamics simulations are performed using Lennard–Jones potentials on a triangular lattice. These simulations represent the first systematic study of the dependence of intrinsic grain boundary mobility on misorientation. The experiments focus on high purity Al, with 〈111〉 tilt boundaries, which are isomorphic to those examined in the simulations. Excellent agreement between simulations and experiments was obtained in almost all aspects of these studies. The boundary velocity is found to be a linear function of the curvature and the mobility is observed to be an Arrhenius function of temperature, as expected. The activation energies for boundary migration varies with misorientation by more than 40% in the simulations and 50% in the experiments. In both the simulations and experiments, the activation energies and the logarithm of the pre-exponential factor in the mobility exhibited very similar variations with misorientation, including the presence of distinct cusps at low Σ misorientations. The activation energy for boundary migration is a logarithmic function of the pre-exponential factor in the mobility, within a small misorientation range around low Σ misorientations.

Measurements of grain boundary mobility during recrystallization of a single-phase aluminium alloy

A combination of in situ annealing and electron backscattered diffraction in the SEM has been used to determine the mobility of high angle grain boundaries in a deformed single-phase Al–Si alloy. It is found that the boundary velocity is directly proportional to the driving pressure and that the activation energy for boundary migration over all the conditions investigated is consistent with control by lattice diffusion of the solute. It is confirmed that tilt boundaries of recrystallized grains misoriented by 40±10° about axes within ±10° of 〈111〉 have an increased mobility compared to other high angle boundaries, whereas the mobilities of 40°〈111〉 twist boundaries are similar to those of general high angle boundaries. The mobility maximum for the 40°〈111〉 tilt boundaries is very broad, which is in contrast to the sharp mobility peaks reported for curvature-driven grain growth, and possible reasons for these differences are discussed.

Investigation of 63Ni diffusion along stationary and moving grain boundaries in Nb

Grain boundary diffusion of 63Ni in high purity polycrystalline niobium has been studied in the temperature range 973–1258 K by the radiotracer serial sectioning technique. Since the grain size in the material could not be completely stabilized by pre-diffusion heat treatment, some part of the grain boundaries could migrate during the diffusion anneals. The tracer penetration profiles were treated using the model of diffusion in polycrystals containing both stationary and moving grain boundaries developed in our previous work. Along with the grain boundary diffusivity, also the average velocity of moving grain boundaries and the activation energy of boundary migration in Nb were determined from the effect of boundary motion on the profile shape. Grain boundary diffusion of Ni in Nb is anomalously fast and exhibits an activation energy of 155.2 kJ mol −1. This behaviour is well consistent with that of Co diffusion in Nb studied previously and implies that diffusion of both elements in Nb is governed by a combined atomic mechanism involving both vacancies and interstitials.

鋼溶接熱影響部の結晶粒微細化に及ぼすＴｉ及びＮの複合添加の影響

Suppressive effects of Ti and N to the austenite grain growth in weld heat affected zone of steels were investigated from a standpoint of the time exponent, n and the activation energy for boundary migration, Q in isothermal grain growth test. The results obtained are as follows. 1) Austenite grain growth of steels added together both Ti and N can be expressed by Beck's equation. 2) Value of n is remarkably influenced by the quantity of a insoluble Ti (or N) and independent of a soluble Ti (or N). 3) Value of Q is increased considerably in duplex addition compared with separate addition of Ti and N. In this case an extent of the effect of a soluble Ti (or N) to Q is a greater than a insoluble Ti (or N). 4) Grain size of austenite isothermaly heated can be estimated using coefficient of austenite grain growth which is decided by quantity of Ti and N added in steels.

Annealing of polycrystalline Au and Au-Ag thin films

Crystal growth by annealing has been observed by both electron microscopy and electron diffraction in thin films of pure gold, pure silver and gold-silver alloys. Changes in grain diameter from values less than 100 Å to 10 4 Å have been observed on annealing. The films were obtained by vacuum evaporation on formvar substrates. The experimental results show different average sizes of the grains for the different alloys and minimum average sizes for a Au-52 at% Ag alloy. The alloys show also different amount of (111) texture. The apparent activation energy for boundary migration has been measured for pure gold films and its value is 0.14 eV. The growth process may be partly accounted for by the grain growth process and by the influence of impurities on the nucleus formation in the first stage of deposition.